Certain fuel cell stack assemblies have to operate at high temperatures. One example of a high temperature fuel cell assembly is a solid oxide fuel cell assembly. Currently the main variants of the solid oxide fuel cell are the tubular solid oxide fuel cell (T-SOFC), the planar solid oxide fuel cell (P-SOFC) and the monolithic solid oxide fuel cell (M-SOFC).
The tubular solid oxide fuel cell comprises a tubular solid oxide electrolyte member which has inner and outer electrodes. Typically the inner electrode is the cathode and the outer electrode is the anode. An oxidant gas is supplied to the cathode in the interior of the tubular solid oxide electrolyte member and a fuel gas is supplied to the anode on the exterior surface of the tubular solid oxide electrolyte member. (This arrangement may be reversed). The tubular solid oxide fuel cell allows a simple cell stacking arrangement and is substantially devoid of seals. However, the fabrication of this type of solid oxide fuel cell is very sophisticated, manpower intensive and costly. Also this type of solid oxide fuel cell has a relatively low power density due to long current conduction paths through the relatively large diameter tubular cells.
The monolithic solid oxide fuel cell has two variants. The first variant has a planar solid oxide electrolyte member which has electrodes on its two major surfaces. The second variant has a corrugated solid oxide electrolyte member which has electrodes on its two major surfaces. The monolithic solid oxide fuel cell is amenable to the more simple tape casting and calendar rolling fabrication processes and promises higher power densities. This type of solid oxide fuel cell requires the co-sintering of all the fuel cell layers in the monolith from their green states. However, this results in serious shrinkage and cracking problems. This type of solid oxide fuel cell is not so easy to manifold and seal.
The planar solid oxide fuel cell is also amenable to tape casting and calendar rolling fabrication processes. Currently it requires thick, 150-200 microns, self-supported solid oxide electrolyte members which limit performance. The planar solid oxide fuel cell also has limited thermal shock resistance.
Solid oxide fuel cells require operating temperatures of around 700 to around 1000° C. to achieve the required electrolyte performance within the active fuel cells.
The operating temperature of a solid oxide fuel cell assembly is in principle high enough for steam reforming of a hydrocarbon fuel internally of the solid oxide fuel cell stack. Internal steam reforming would simplify the balance of plant of a solid oxide fuel cell assembly (or solid oxide fuel cell stack) and improve operating efficiency. However, reforming of a hydrocarbon fuel within the solid oxide fuel cell stack has a number of problems which have not been overcome. Full internal reforming of the hydrocarbon fuel in solid oxide fuel cell stacks is precluded by the strongly endothermic nature of the steam reforming reaction, and consequential thermal shocking of the delicate fuel cells. Internal reforming on nickel cermet anodes in solid oxide fuel cells tends to catalyse carbon formation.
Embodiments of the present invention seek to provide a novel fuel cell assembly having good thermal and mechanical compliance during cold start up to normal operating temperatures.
EP0668622B1 discloses a solid oxide fuel cell which comprises a plurality of modules. Some of these modules comprise hollow members, which have two parallel flat surfaces upon which the solid oxide fuel cells are arranged. The opposite ends of each module are connected to reactant manifolds by compliant bellow connections.
However, such an arrangement does not provide sufficient thermal and mechanical compliance in the solid oxide fuel cell stack to minimise the mechanical and thermal stresses in the solid oxide fuel cell stack.
EP1419547B1 discloses a solid oxide fuel cell stack, which comprises a plurality of modules, the modules comprising elongate hollow members, the hollow members having a passage for flow of reactant. The modules are arranged so that at least one end of each module is connected to an end of an adjacent module to allow reactant to flow sequentially through the modules in a serpentine type arrangement.
However, the arrangement only provides thermal and mechanical compliance within a bundle and does not prevent thermal and mechanical stresses building up across adjacent bundles. Furthermore, the arrangement does not address issues with fuel distribution from fuel cell tubes throughout a stack.